refactor: Create and use generic portal traversal utility

[marcustyphoon]

Description

A little messy, but it works? Maybe helpful for #1931? I dunno. Possibly not worth it otherwise?

This:

• Places both xkit-injection-request and xkit-injection-response handlers on document.documentElement (yes, this could mean a few extra handlers firing once in a blue moon; one could make xkit-injection-element-response and add the event listener twice to prevent this I guess but I don't think that would save any cycles edit: see comment)
• Allows injected functions to return DOM elements by passing them from the main world to the isolated world in the same way we pass the target DOM element from the isolated to the main world: event bubbling.
• Uses this ability to replace the overly-specific injected function in the meatballs menu util with, essentially a version of .closest() that traverses portals.

Testing steps

• Confirm that meatballs menu items are still added to post meatball menus.

More tests could be run with #677/#1662, I guess.

[AprilSylph]

Yeah, separate event names would be better. As it is, the code diff for src/main_world/index.js just looks like it's trying to avoid setting { bubbles: true } when dispatching events to document.documentElement, which... there's nowhere for it to bubble anyway... so it doesn't make sense until you realise it's just being used as a signal for responseHandler to resolve to something different. Best to just be explicit instead.

[marcustyphoon]

The remaining parts of the above diff:

diff --git a/src/main_world/index.js b/src/main_world/index.js
index 08508f7e..0e0a5858 100644
--- a/src/main_world/index.js
+++ b/src/main_world/index.js
@@ -22,7 +22,7 @@ document.documentElement.addEventListener('xkit-injection-request', async event
         new CustomEvent('xkit-injection-element-response', { detail: JSON.stringify({ id }), bubbles: true })
       );
     } else {
-      document.documentElement.dispatchEvent(
+      target.dispatchEvent(
         new CustomEvent('xkit-injection-response', { detail: JSON.stringify({ id, result }) })
       );
     }
diff --git a/src/utils/inject.js b/src/utils/inject.js
index b29e2953..1591935a 100644
--- a/src/utils/inject.js
+++ b/src/utils/inject.js
@@ -17,7 +17,7 @@ export const inject = (path, args = [], target = document.documentElement) =>
       const { id, result, exception } = JSON.parse(detail);
       if (id !== requestId) return;
 
-      document.documentElement.removeEventListener('xkit-injection-response', responseHandler);
+      target.removeEventListener('xkit-injection-response', responseHandler);
       document.documentElement.removeEventListener('xkit-injection-element-response', responseHandler);
 
       if (exception) {
@@ -28,7 +28,7 @@ export const inject = (path, args = [], target = document.documentElement) =>
         resolve(result);
       }
     };
-    document.documentElement.addEventListener('xkit-injection-response', responseHandler);
+    target.addEventListener('xkit-injection-response', responseHandler);
     document.documentElement.addEventListener('xkit-injection-element-response', responseHandler);
 
     target.dispatchEvent(